1. Field of the Invention
The present invention relates to mobile communications devices such as cell telephones and to methods for determining accurately and precisely the positions of such devices within a network area. Cell telephones can therefore be located in order to dispatch emergency police or medical assistance to a caller or for any other purpose.
2. Background of the Invention
New rules in the United States require that by 2001 mobile cellular telephones must be located, based solely on the location of the point of origination of their transmissions. The initial standard is that the cellular telephones in any given network area making an emergency call must be located accurately with a root mean square error less than 125 meters. It would be preferred to implement this standard without retro-fitting existing cellular phones.
Existing base stations in Industry Standard ("IS") - 54 and 136 TDMA systems generally employ location verification modules ("LVM"s) that verify data in mobile station voice transmissions at the time of handover from one base station to another. For a TDMA call to proceed, the timing of the mobile station and the base station must be closely synchronized. Under IS-54 and -136, the mobile station provides, or can be requested to provide, a "shortened burst" transmission at the time the call is set up or for any handoff from one base station to another in the network. Under IS-54, using an analog control channel, the shortened bursts are transmitted by default. The IS-136 control channel is digital and a shortened burst must be requested by the base station. Shortened bursts are time slot signal transmissions of up to about 5.58 milliseconds each, which repeat every 20 milliseconds for a period of not more than 5 seconds. These bursts are used to synchronize time slotted voice transmissions between the mobile unit and base stations. In accordance with the invention, these bursts are also well suited for use in accurately detecting the position of the mobile unit.
The shortened bursts are used to measure quantities related to the distance of the mobile unit from base stations in its geographical area. The measured quantities are correlated with known triangulation techniques to locate the x, y coordinate position of the mobile unit.
Global positioning systems have been used with a mobile receiver that determines its position by detecting the time of arrival of signals transmitted by satellites. Such systems are not employed to detect the position of a mobile transmitter in relation to spatially disposed base stations.
A system has also been developed to locate the position of a mobile cell telephone unit by using the time difference of arrival at base stations of signal bursts provided in the reverse control channel by the mobile unit in initiating a call. In this system the reverse control channel is not available for locating the mobile unit during the call and therefore the position of the mobile unit can be determined only when the call is initiated. The reverse control channel also provides a limited time for accessing position data and therefore it cannot determine a position with accuracy over a substantial time.
The system of the invention uses traffic channel bursts such as the shortened burst to determine the position of the mobile unit. The traffic channel bursts can be used to detect the position of the mobile unit when the call is being set up and for the duration of the call. The system can therefore track the position of a moving unit as the call progresses and can initiate a determination of position at any time during a call. The system can also obtain position data over a substantial time and can therefore achieve a precise location of position. Moreover, the shortened traffic channel burst was designed to be used for adjusting the timing of time slots and it is therefore well suited to make time of arrival signal measurements that are used in locating the position of the mobile unit.
It is therefore an object of the invention to provide an accurate method for determining the position of a mobile cell telephone unit in existing cell communication systems without requiring additional or modified hardware for the mobile units in such systems.
It is another object of the invention to provide a system for locating the position of a mobile cell telephone unit during the set up of a mobile call and during the pendency of the call.
It is a further object of the invention to provide such a system which uses the traffic channel shortened burst to locate the mobile unit with respect to spatially disposed base stations in the system.
Another object of the invention is to provide a system wherein the position of the mobile unit is automatically located at the beginning of a call in response to emergency conditions.
A further object of the invention is to provide a system that checks the accuracy of the position locating information for a mobile unit and continues to improve the accuracy of the location of position, if required.
Another object of the invention is to provide a system that allows continued monitoring of the position of a mobile unit as the unit moves.
These and other objects of the invention will become apparent from a review of the following description, drawings and claims.